Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a process of producing IC, LSI etc., in order to provide reliable semiconductor connections for various coatings used in manufacturing the circuits in each steps of stacking an oxide layer or metal layer thereon. In the step of polishing finish, a polishing pad is generally stuck on a rotatable supporting disk called a platen, while a workpiece such as a semiconductor wafer is stuck on a polishing head. By movement of the two, a relative speed is generated between the platen and the polishing head while polishing slurry having abrasive grains is continuously supplied to the polishing pad, to effect polishing processing.
As polishing characteristic of a polishing pad, it is requested that a polishing object is excellent in planarity and in-plane uniformity and a polishing rate is large. A planarity and in-plane uniformity of a polished object can be improved to some extent with a polishing layer higher in an elastic modulus.
In view of development in next generation elements, a polishing pad having a higher hardness which can further improve planarity is required. In order to improve planarity, it is possible to use a non-foam-based hard polishing pad. However, when such the hard pad is used, there arises a problem that a surface to be polished of a polishing object is scratched. In addition, since the non-foam-based polishing pad cannot sufficiently retain abrasive grains in slurry on a pad surface at polishing processing, this is not desirable also from a viewpoint of a polishing rate.
In addition, a polishing pad in which a water-soluble material is dispersed in a thermoplastic polymer has been disclosed (Japanese Patent Application Laid-open (JP-A) No. 2001-47355). Although this polishing pad is a non-foam, since a water-soluble material dispersed in a polishing pad is dissolved at polishing, a pore like a foam is formed on a polishing pad surface, a polishing pad is swollen, and a hardness of a polishing pad surface is reduced, the pad is effective for reducing a scratch and improving a polishing rate. However, since a surface of the polishing pad is swollen, and a hardness is reduced, planarization property is insufficient.
In addition, for the purpose of realizing both of improvement in planarity and reduction in scratch, there is disclosed a polishing pad comprising a polymer of an isocyanate-terminal prepolymer obtained by reacting a high-molecular-weight polyol containing an organic polyisocyanate and a water-soluble polyol and a low molecular polyol, and a chain extender (Japanese Patent No. 3571334). However, since a surface of the polishing pad is swollen, and a hardness is reduced, planarization property required in the future cannot be sufficiently satisfied.
On the other hand, a polishing rate can be improved by using a foam containing pores, thereby, increasing an amount of slurry to be retained.
As a polishing pad satisfying the aforementioned properties, a polishing pad comprising a polyurethane resin foam has been disclosed (JP-A No. 2000-17252 and Japanese Patent No. 3359629). The polyurethane resin foam is produced by reacting an isocyanate-terminal prepolymer and a chain extender (curing agent) and, as a high-molecular-weight polyol component of an isocyanate prepolymer, polyether (particularly polytetramethylene glycol having a number average molecular weight of 500 to 1600) and polycarbonate polyol are used as a suitable material, from a viewpoint of hydrolysis resistance, elasticity property, and abrasion resistance.
However, when a modulus of a polishing layer is increased (increase in a hardness) for improving planarization property of a polishing pad, a specific gravity is increased and, as a result, there is a problem that the number of pores per unit area is decreased, and a polishing rate is reduced.
In addition, as a method of increasing an amount of slurry to be retained, there is a method of rendering a polishing pad itself hydrophilic, specifically, (1) a method of introducing a hydrophilic group such as a hydroxyl group etc. into a matrix material, and (2) a method of mixing a matrix material and a hydrophilic material. For example, a composition for a polishing pad containing (A) a crosslinking elastomer and (B) a material having a functional group such as a hydroxyl group etc. is disclosed (JP-A No. 2002-134445). In addition, a polishing equipment obtained by further adding a hydrophilic material, to a material constituting a polishing equipment, or adding a hydrophilic group thereto (modification) is disclosed (JP-A No. 2003-11066). In addition, a polishing pad comprising a thermosetting polymer matrix resin which is hydrophilic, and contains a substantially water-insoluble sheet is disclosed (JP-A No. 2002-59358). Further, a polishing pad comprising a polyurethane composition containing a urethane resin obtained by copolymerizing a compound having a hydrophilic group, and containing a hydrophilizing agent is disclosed (JP-A No. 2003-128910).
However, in the (1) method, when a matrix material is polyurethane, a hydrophilic group containing active hydrogen of a hydroxyl group etc. is reacted with an isocyanate group upon synthesis of polyurethane and, as a result, an unreacted polyol component may remain in a material. And, there is a tendency that since this remaining polyol component exerts the plastic effect, physical property of a polishing pad is deteriorated. In addition, in the (2) method, it is difficult to uniformly mix a hydrophilic material into a matrix material, and a polishing pad having uniform physical property cannot be obtained.
On the other hand, if a polishing rate varies during from immediately after use to completion of use, polishing condition must be adjusted and there is a problem that a polishing efficiency is worse.
For example, for the purpose of providing a non-foam urethane polishing material which can effectively polish a semiconductor wafer and is excellent in planarity, there is disclosed a polishing material comprising a polishing material composition which comprises an isocyanate-terminal urethane prepolymer and an active hydrogen-containing compound and in which the isocyante-terminal urethane prepolymer is obtained by using, as polyisocyante, aromatic diisocyante, and using a polyol component consisting of a high-molecular-weight polyol and a low-molecular polyol, the low-molecular polyol in the polyol component being diethylene glycol, 1,3-butylene glycol etc. (JP-A No. 2000-17252).
In addition, for the purpose of imparting dressing property to a polishing cloth itself and prolonging a polishing life, a polishing cloth comprising a polyurethane composition and having an abrasion amount by a Taber abrasion test of 150 to 350 mg is disclosed (JP-A No. 2001-277101).
However, the polishing material described in JP-A No. 2000-17252 comprises a non-foam urethane, and since such the non-foam-based polishing material has a low polishing rate, a groove is foamed on a polishing surface, and it is very difficult to stabilize a polishing rate due to local presence of abrasive grains and polishing swarf in slurry. In addition, since the polishing cloth described in JP-A No. 2001-277101 is easily abraded, and has a low hardness (due to not uniform pores, and a large pore size), planarity and in-plane uniformity are not sufficient, and a greater change in a polishing rate cannot be avoided.
Further, the previous polishing pad with a groove has a problem that a groove is clogged with abrasive grains or polishing swarf during polishing, and a polishing rate is reduced during use.
In addition, Japanese Patent Application National Publication (Laid Open) No. 2001-513450, Japanese Patent Application National Publication (Laid Open) No. 2001-518852 and Japanese Patent Application National Publication (Laid Open) No. 2002-535843 disclose a polishing pad comprising a polishing material having a breaking extension of 500% or lower. However, technical meaningfulness of use of a pressing material having a breaking extension of 500% or lower is not described at all.